Fluidized bed catalytic cracking (commonly referred to as FCC) processes were developed during the 1940's to increase the quantity of naphtha boiling range hydrocarbons which could be obtained from crude oil. Fluidized catalytic cracking processes are now in widespread commercial use in petroleum refineries to produce lighter boiling point hydrocarbons from heavier feedstocks such as atmospheric reduced crudes or vacuum gas oils. Such processes are utilized to reduce the average molecular weight of various petroleum-derived feed streams and thereby produce lighter products, which have a higher monetary value than heavy fractions. Though the feed to an FCC process is usually a petroleum-derived material, liquids derived from tar sands, oil shale or coal liquefaction may be charged to an FCC process. Today, FCC processes are also used for the cracking of heavy oil and reduced crudes. Although these processes are often used as reduced crude conversion, use of the term FCC in this description applies to heavy oil cracking processes as well.
The operation of the FCC process is well known to those acquainted with process for upgrading hydrocarbon feedstocks. Differing designs of FCC units may be seen in the articles at page 102 of the May 15, 1972 edition and at page 65 of the Oct. 8, 1973 edition of "The Oil & Gas Journal". Other examples of FCC processes can be found in U.S. Pat. No. 4,364,905 (Fahrig et al.); U.S. Pat. No. 4,051,013 (Strother); U.S. Pat. No. 3,894,932 (Owen); and U.S. Pat. No. 4,419,221 (Castagnos, Jr. et al) and the other FCC patent references discussed herein.
A majority of the hydrocarbon vapors that contact the catalyst in the reaction zone are separated from the solid particles by ballistic and/or centrifugal separation methods. However, the catalyst particles employed in an FCC process have a large surface area, which is due to a great multitude of pores located in the particles. As a result, the catalytic materials retain hydrocarbons within their pores and upon the external surface of the catalyst. Although the quantity of hydrocarbon retained on each individual catalyst particle is very small, the large amount of catalyst and the high catalyst circulation rate which is typically used in a modern FCC process results in a significant quantity of hydrocarbons being withdrawn from the reaction zone with the catalyst.
Therefore, it is common practice to remove, or strip, hydrocarbons from spent catalyst prior to passing it into the regeneration zone. It is important to remove retained spent hydrocarbons from the spent catalyst for process and economic reasons. First, hydrocarbons that entered the regenerator increase its carbon-burning load and can result in excessive regenerator temperatures. Stripping hydrocarbons from the catalyst also allows recovery of the hydrocarbons as products. The most common method of stripping the catalyst passes a stripping gas, usually steam, through a flowing stream of catalyst, countercurrent to its direction of flow. Such steam stripping operations, with varying degrees of efficiency, remove the hydrocarbon vapors which are entrained with the catalyst and hydrocarbons which are adsorbed on the catalyst.
The efficiency of catalyst stripping has been increased by using a series of baffles in a stripping apparatus to cascade the catalyst from side to side as it moves down the stripping apparatus. Moving the catalyst horizontally increases contact between it and the stripping medium. Increasing the contact between the stripping medium and catalyst removes more hydrocarbons from the catalyst. As shown by U.S. Pat. No. 2,440,625, the use of angled guides for increasing contact between the stripping medium and catalyst has been known since 1944. In these arrangements, the catalyst is given a labyrinthine path through a series of baffles located at different levels. Catalyst and gas contact is increased by this arrangement that leaves no open vertical path of significant cross-section through the stripping apparatus. Further examples of similar stripping devices for FCC units are shown in U.S. Pat. Nos. 2,440,620; 2,612,438; 3,894,932; 4,414,100; and 4,364,905. These references show the typical stripper arrangement having a stripper vessel, a series of baffles in the form of frusto-conical sections that direct the catalyst inward onto a baffle in a series of centrally located conical or frusto conical baffles that divert the catalyst outwardly onto the outer baffles. The stripping medium enters from below the lower baffle in the series and continues rising upward from the bottom of one baffle to the bottom of the next succeeding baffle. Variations in the baffles include the addition of skirts about the trailing edge of the baffle as depicted in U.S. Pat. Nos. 2,994,659 and 2,460,151; the use of multiple linear baffle sections at different baffle levels as demonstrated by FIG. 3 of U.S. Pat. Nos. 4,500,423 and 5,019,354; and the use of orifice openings in stopper skirts as shown in U.S. Pat. No. 5,015,363. A variation in introducing the stripping medium is shown in U.S. Pat. No. 2,541,801 where a quantity of fluidizing gas is admitted at a number of discrete locations.
Although the frusto-conically shaped stripped grids operate well in most FCC applications, they present a disadvantage when the stripper vessel becomes large. In order for these frusto-conical baffles to operate correctly, the baffle must have a downward sloping angle of approximately 45.degree.. As the capacity of the FCC unit increases, so does the catalyst throughput passing downwardly in the stripper. The higher catalyst throughput dictates a relatively large cross-sectional area for the stripper vessel. The required cross-sectional area of the stripper vessel in combination with the 45.degree. angle on the stripper grids greatly extends the length of the stripper as the capacity becomes larger. Since the area under the stripper grids is devoid of the catalyst particles, this large area goes essentially unused. As designers of FCC units strive to achieve more efficient stripping, the number of stages, i.e., grids in the stripper vessel have continued to increase. Therefore, the combination of an increasing number of grids to provide additional stages of stripping and larger diameters for the stripping vessel, greatly increase the length and cost of traditional FCC strippers that use the traditional frusto-conical baffle design.
It is an object of this invention to provide a stripping process that uses a compact grid design to reduce the stripper height requirements while maintaining the same or better stripping efficiency through the stripper vessel.
It is a further object of this invention to provide stripper grids that require a relatively small length for a highly efficient stage of stripping while also permitting access through the stripper for inspection and maintenance.